Various processes are known for compacting powdery or granular materials for the purpose of manufacturing casting molds made of sand.
U.S. Pat. No. 3,170,202 describes a compacting process which uses gas pressure. A gas mixture is ignited to create an exothermic reaction. The resulting gas pressure then compacts the materials, resulting in a sand mold.
German Patent 1,097,622 describes a compacting process in which the compacting of the materials occurs by releasing the pressure of a highly compressed gas.
Compressed gas molding devices which are used for performing these compacting processes operated exclusively on the principle of acceleration compression. A pressure surge is generated, acting very rapidly on the mass of material, and also accelerating it. Compaction occurs as a result of the compression of the granular or powdery material at the pattern. Ideally, each sand particle is affected by the pressure wave and the absorbed acceleration energy is transferred to each successive powder layer until the pattern is reached. This would result in a uniformly compacted shape with a high degree of hardness, the resulting mold being usable to manufacture precise, high-quality castings.
Clay-like molding powders, which must be transported from the preparation station to the mold box in the manufacturing plant, are most often used for compacting purposes.
When the material mass arrives at the mold box, it is initially not a completely homogenous mass because lumps form during transportation as a result of collisions which cause some compacting. The collisions can be traced to the tendency of the mass to overcome the relative difference in heights which must be overcome during the lengthy transportation to the mold box.
Furthermore, the need for efficient production requires that the pattern plates be occupied to a considerable degree. This leads to the fact that the clearance between the mold box wall and the pattern becomes even smaller. In order to manufacture a usable and high quality mold, however, these interstices which become even smaller must be homogeneously filled with sand. But this is not always possible for the reasons described above so that there is already a predisposition toward undesired bridge-building across these interstices when the matrial mass is poured into the mold box.
When the pressure surge is applied to compact the material mass, the pressure wave takes effect with equal force on the bridge areas. Because the compression forces have a uniform effect on the support areas of the bridges, they are made firmer in their respective positions.